Adjust Overal Brightness in Cycles
Finally figured this out. Basically, you should use the 'exposure' slider in the color management tab to make overall brightness adjustments while keeping the light 'ratio' the same. Seems obvious, but by default Cycles uses strong clamping which botches this up. If you disable clamping, you can render once then adjust lightness through the exposure slider with identical results to re-rendering with brighter light sources.